FAA Powerplant Written Practice Test

What will happen if the vapor vent float in a pressure injection carburetor loses buoyancy and sinks deeper in its chamber?

1. The fuel flow will stop completely

2. The fuel returning to the tank will be reduced

3. The amount of fuel returning to the fuel tank from the carburetor will be increased

4. The mixture will become richer

The correct answer is: The amount of fuel returning to the fuel tank from the carburetor will be increased

If the vapor vent float in a pressure injection carburetor loses buoyancy and sinks deeper in its chamber, the increased sinking of the float allows for a greater opening of the venturi or a pathway for fuel to flow. As a result, more fuel can be allowed to flow into the system, increasing the amount of fuel returning to the tank from the carburetor. In a pressure injection carburetor, the vapor vent float is designed to regulate the level of fuel within the chamber in response to vapor versus liquid fuel conditions. When the float sinks due to loss of buoyancy, it signals that the fuel level is not appropriately balanced, leading to the carburetor's mechanisms compensating, which results in an increase in the overall fuel pressure and typically allows for more fuel to be pushed back to the tank. Understanding this mechanism emphasizes the relationship between float position, fuel flow regulation, and fuel efficiency in carburetors. It showcases the importance of buoyancy in maintaining the correct fuel-air mixture for proper engine function.